Method of forming electrical contact materials

ABSTRACT

ELECTRICAL CONTACT DEVICE IS FABRICATED BY SPRAYING FINE PARTICLES OF ARC RESISTANT MATERIAL (E.G., WC) AGAINST SURFACE OF CONDUCTIVE METAL (E.G., AU OR AG) IN A MANNER SUCH THAT PARTICLES PENETRATE, AND BECOME EMBEDDED IN, THE CONDUCTIVE METAL. CONDUCTIVE METAL AGAINST WHICH PARTICLES ARE SPRAYED MAY BE OF BUTTON OR SOLID CONDUCTIVE METAL OR MAY COMPRISE COATING OR PLATING OF CONDUCTIVE METAL ON CONTACT DEVICE SUCH AS SWITCH ARM OR ELECTRICAL TERMINAL.

Oct. 10,1972 w. J JACOBS ETAL 3,697,389

METHOD OF FORMING ELECTRICAL CONTACT MATERIALS Original Filed Jan. 2, 1968 United States Patent Ofice I 3,697,389 Patented Oct. 10, 1972 3,697,389 METHOD OF FORMING ELECTRICAL CONTACT MATERIALS Wayne John Jacobs and Robert William Fritz, Camp Hill, and Howard Richard Peilfer, New Cumberland, Pa., assignors to AMP Incorporated, Harrisburg, Pa. Continuation of application Ser. No. 695,123, Jan. 2, 1968. This application Apr. 23, 1970, Ser. No. 31,188 Int. Cl. C23b 5/52 US. Cl. 204-37 R 4 Claims ABSTRACT OF THE DISCLOSURE Electrical contact device is fabricated by spraying fine particles of are resistant material (e.g., WC) against surface of conductive metal (e.g., All or Ag) in a manner such that particles penetrate, and become embedded in, the conductive metal. Conductive metal against which particles are sprayed may be of button or solid conductive metal or may comprise coating or plating of conductive metal on contact device such as switch arm or electrical terminal.

BACKGROUND OF THE INVENTION This application is a continuation of application Ser. No. 695,123, filed Jan. 2, 1968, and now abandoned.

A commonly known type of contact material comprises a composite of conductive metal, such as gold or silver, which forms a matrix and a dispersed phase of are resistan-t particles such as tungsten, molybdenum, their carbides, or oxides, the term are resistant in this context being understood to mean resistant to the damage caused by arcing. Contact materials of this type are usually produced by conventional powder metallurgy techniques, that is, by mixing powders of the metal and the are resistant material and pressing and sintering the resultant mixture. These contacts are usually formed as relatively massive buttons or blocks which are welded or brazed to a backing member. Contact materials of this type are highly advantageous for the reason that they have electrical conducting properties which are comparable with the conducting properties of the matrix metal and they also have good are resistance and are long lived.

Heretofore, it has not been practical to provide composite conducting materials of the general type described above on inexpensive parts such as small switch arms or electrical terminals. Switch arms for small inexpensive switches are usually produced at a very low cost by conventional die stamping and forming operations. Parts of this type require a good contact surface and the contact surface is generally achieved by simply electrodepositing -a conductive metal such as gold, silver, or tin on the surface of the part, the thickness of the deposit being kept to a minimum level for economic reasons. It would, in many instances, be desirable to have a composite contact material on a switch arm or a terminal but the cost of providing the composite surface and the irnpracticality of forming an extremely thin composite contact surface have precluded achievement of this type of surface.

The present invention is directed generally to the problem of providing a relatively thin layer of contact material of the conductive metal and hard particle type on electrical parts, such as switch arms or contact terminlals, which must be manufactured at a relatively low cost. The invention also finds application in production of relatively massive contact members of the type which heretofore have been manufactured by conventional powder metallurgy techniques, for example, contact buttons used in relays or circuit breakers which must withstand high voltage and high currents and which must be resistant to arcing and wear.

tlt is an object of the invention to provide an improved contact material. A further object is to provide an improved method for forming contact materials. A more specific object is to provide a method of applying composite contact materials to the surfaces of relatively small inexpensive electrical parts, such as switch arms or electrical terminals. A further object is to provide a method of forming composite contact materials at a reduced cost and at production rates which are comparable to the production rates achieved in conventional die stamping and forming operations.

These and other objects of the invention are achieved in one prefered embodiment thereof comprising a conductive metal matrix such as gold containing about 20 to 50 volume percent of fine particles such as tungsten carbide. Depending upon the thickness of the gold, the particles may be concentrated in only the surface portions of the gold matrix or they may be dispersed throughout the gold. Contacts in accordance with the invention are fabricated by heating the gold matrix to an elevated temperature, preferably in the range of about 500-1,100 IR, and directing a stream of fluid, preferably an inert gas such as nitrogen, against the surface of the gold. This inert gas stream carries the are resistant particles and causes them to impinge upon the surface of the matrix with a velocity such that they penetrate the surface and some of the particles become embedded or implanted entirely in the matrix metal while others merely become embedded in the surface. The matrix material, the gold or silver, may take the form of a relatively thin coating such as an electrodeposited plating on a switch arm or the like. Alternatively, the matrix material may be in the form of a relatively massive button which is adapted to be soldered or brazed to the switch arm of other member to which it is to be applied.

In the drawings:

FIG. 1 is a perspective view of one form of apparatus for producing contact materials in accordance with the invention;

FIG. 2 is a perspective view of a switch blade provided with a contact material in accordance with the invention;

FIG. 3 is a side view of a typical reed switch provided with switch blades of the type shown in FIG. 2; and

FIG. 4 is a perspective view of a relay arm having a contact button thereon.

The invention will first be described with reference to a specific embodiment comprising contact member in the form of a reed switch blade 4 as shown in FIG. 2. A general discussion of the invention and other applications thereof is set forth following the description of the specific embodiment.

A conventional reed switch as shown in FIG. 3 comprises a glass envelope 12 having a pair of blades 4 extending through its opposite ends with the end portions 6 of the blades 4 overlapping. The blades 4 have reduced intermediate portions 8 which are sealed to the glass envelope 12 and terminal portions 10 which extend beyond the glass envelope, the connections to the external circuity being made by these terminal portions. The contact end portions 6 of the blades 4 move against each other when the switch is closed under the influence of a magnetic field which is established by a coil 13 surrounding the envelope 12. Switches of this type are widely used in the communications field and elsewhere and must be capable of hundreds of thousands or even millions of openings and closings with a high degree of reliability. The blades 4 must be of a highly magnetic material such as nickel or a nickel alloy. Since nickel is not a good contact material (although it is a reasonably good conductor) it is necessary to provide a separate contact material on the ends 6 of the blades. Because switches of this type must be relatively inexpensive it has been common practice in the past to electroplate a thin layer of gold on the contact surfaces 6 in order to achieve adequate contact properties although it would be desirable to provide acomposite contact material such as gold containing arc resistant particles rather than pure gold. It should be added that the thickness of the gold or other contact material must be kept to a minimum level for the additional reason that the gold interferes with the magnetic behavior of the blade and a thick layer of gold could interfere withthe operation of the switch.

In accordance with the present invention, a composite contact surface is provided on the end 6 of the blade 4 by means of an apparatus 14 which impregnates a gold layer on the end of the blade with fine are resistant particles. This apparatus is mounted on a suitable frame 16 and comprises a chamber 18 .into which hard arc resistant particles are charged through an inlet 20. A stream of inert gas, such as nitrogen, is introduced into the chamber 18 by means of a tube 26 and passes through the chamber where the particles are entrained in the stream. The gas stream flows from the chamber 18 through a tube 22 which has a nozzle 24 on its end, this nozzle being located above a supporting surface 28 of a heating element 30. Advantageous, the, carrier gas is heated to a temperature of about 500 F. and it is maintained at its elevated temperature by means of heating coils indicated at 32 which surround the tube 22. Suitable insulation 34 may be provided on the surface of the tube and in surrounding relationship to the heating coils if desired.

The blades 4 are preferably manufactured by stamping and forming methods and the end portions 6 of the blades 4 are provided with a thin plating of gold or other precious metals by electroplating methods. For economic reasons, the thickness of this plating of gold will be kept to a minimum level, for example, 100 millionths of an inch. The individual contact members 4 are placed on the support surface 28. and heated during implantation to a temperature in the range of about 5O0--1,l00 F., the higher temperatures being preferable for best results. The gas stream.

does not flow continuously but is controlled by a suitable valve which is opened by the operator when a blade 4 is being treated. The entire operation of implanting the are resistant particlesin the gold takes only a very short time, of the order of a second or two. The pressure of the inert gas and therefore the velocity of the gas stream emanating from the nozzle 24 should be such that the hard are resistant, particles penetrate the gold. -It is preferred, for best results, to have the particles penetrate the surface of the goldto a substantial depth, to about 2% or 3 times the i diameter of the largest particle carried in the gas stream.

While the disclosed form of apparatus for carrying out the invention is adapted to treat only a single contact member 4 at a time, it will be apparent that the method can be carried out continuously, .for example, by feeding a strip of contact members past the nozzle and synchronizing the flow of the. carrier gas with periodic pauses of the strip beneath the nozzle. Thus the contact members, while still in strip form, can be electroplated with gold and can then be passed under the nozzle 24 to implant the hard particles in the surface of the gold.

The matrix metal which is deposited by electrodeposition on the end of the contact member 4 will, as a practical matter, usually be gold although other matrix materials would be satisfactory, for example, silver, tin, platinum, and rhodium. The particles implanted in the matrix'metal may, in general, be any of the hard abrasive and arc resistant particles used in powder metallurgy processes.

Where the end 6 of the contact member is of gold or is plated with gold, the use of tungsten carbide is particularly advantageous for the reason that tungsten carbide appears to be compatible with gold and is capable of being wetted by gold. If a silver plating is provided on the contact members, cadmium oxide particles can be used. As

grade of tungsten carbide particles (Firth/Sterling Company of Pittsburgh, Pa. Type A) has an average maximum dimenson of about one micron and has been used with a high degree of success in the practice of the invention. Metallographic examination of contact surfaces prepared with this material reveals that most of .the particles have a maximum dimension in the range of about one-half micron to two microns. A few particles have a maximum dimension which is in excess of two microns, and it can be inferred that particles having a maximum dimension of less than one-half micron are also present although such particles would be beyond the range of optical resolutions and would not be detectable by optical microscopy methods.

The spraying operation can be carried out with the work piece, that is, the plated end of the blade, at room temperature; however, heating of the work piece appears to soften the gold and to facilitate the penetration of the gold by the particles being implanted. A preferred temperature range for the process is about 500 F. to about 1,100 F. At lower temperatures, the particles do not penetrate to the extent that they do at the higher temperatures. Temperatures above 1,100 F. should be avoided for the reason that there is a tendency for the particles to agglomerate While they are entrained in the gas stream. For example, agglomeration has been noted where tungsten carbide particles were being sprayed onto a gold surface and the gold was heated to a temperature of 1,175 F. a

For best results, the contact material should have a minor volume percent of hard particles and a major percent of matrix metal.v In the case of gold tungsten carbide materials, good results are achieved if the tungsten carbide particles constitute about 10 to 20 percent of the total volume and the gold matrix to percent. The concentration .of tungsten carbide particles can, however, be raised to about 40 percent and goodresults will be achieved although the electrical conducting properties of the contact will deteriorate to some extent.

It will be apparent from the foregoing discussion that a salient advantage of the invention is that a composite type contact materialcan be provided at, a low cost on inexpensive electrical parts such as switch arms or elec- 'trical terminals. The thickness of the gold on such parts,

as previously noted, is often limited to about millionths of an inchrfor economic reasons and fine abrasive particles can vbe implanted in platings of this type by the practice of the instant invention. It is not intended to imply, however, that the invention is limited to the intplanation of are resistant particles in electrodeposit layers on contact members only. The invention also finds application to the production of contact buttons of the type shown at 36 (FIG. 4) on a relay arm 38.-iFor example, a button might be cast of gold and placed under the nozzle 24 and the are resistant particles implanted as previously described. This method of fabricating contact buttons offers advantages over the conventional powder metallurgy techniques in that gold powders are usually somewhat more expensive than gold ingots and the implanting process herein described can be carried out at a lower cost than the known pressing and sintering techniques.

Depending upon the thickness of the matrix metal, that is, the gold on the contact member 4, the particlesmay be or may not be concentrated in, or on, the surface layer. Where the gold is of the order of 100 millionths of an inch thick, and the particles have an average diameter of about one micron, the particles will penetrate entirely through the gold and some particles, at least, will be located adjacent to the substrate, that is, the nickel base. The concentration of particles, will, of course, be relatively higher at the surface of the electrodeposited gold, a situation which is desirable rather than objectionable, for the reason that the surface constitutes the actual contact area.

It has been noted, during visual examination of contact surfaces, that failures frequently are associated with defects in the gold coating such as pores, ridges, or cracks. It is noted that such defects or irregularities in a contact surface are at least partially eliminated as a result of the physical efiect of the implanting operation. In other words, the peening or hammering effect appears to close up any pores or cracks which may exist and flatten any ridges on the gold. It is believed that this peening effect thus reduces the incidence of early failures in contacts in accordance with the invention which may stem from imperfect gold plating.

The advantages achieved by the practice of the invention are demonstrated by the specific examples presented below. In general, it can be said thatcontacts in accordance with the invention are superior to precious metal or other conductive metal contacts under many normal conditions of use. Reed switch blades of the type shown in FIG. 2 of the drawing are usually operated at relatively low voltages and currents and the failure of reed switches is usually a result of an increase in the contact resistance of the switch blades. Where the blades are provided with a conventional gold plating, failures will frequently occur after the switches have been cycled about 2,000,000 times while switch blades provided with a gold-tungsten carbide material in accordance with the invention have been known to operate for 100,000,000 cycles and more without a significant number of failures. Contact buttons of the type shown in FIG. 4 are usually provided on electrical devices which operate at relatively high voltages and currents and failure is usually by welding of two contacts against each other or by sticking, that is, incipient welding which prevents rapid opening of the contact. Again, as is shown by the examples presented below, contact materials in accordance with the invention will withstand many more cycles than pure gold or other precious metal contacts. Contacts in accordance with the invention perform comparably to contacts made by conventional powder metallurgy techniques although contacts in accordance with the invention can be produced at a substantial cost saving as compared with powder metallurgy techniques.

EXAMPLE I Reed switch blades of the type shown in FIG. 2 of the drawing were provided with a composite surface in accordance with the invention. The blades were of Niron (52% Ni, 48% Fe) alloy and were electroplated at one of their ends with a hard bright gold plate by means of an acid gold plating solution, the thickness of the gold plating being about 100 10- inches.

The gold plating of the switch arms was impregnated with tungsten carbide particles by placing each switch arm on the heated anvil 28 of the apparatus of FIG. 1. The heated anvil raised the temperature of the switch arm to 700 F., the temperature being determined by a thermocouple. Nitrogen gas at a pressure of 90 lbs/in. was used. The gas was heated by the heating element wrapped around the gas delivery tube (32, FIG. 1).

The valve controlling the flow of nitrogen through the tube 22 was opened four times to produce four spurts of gas directed against the surface of the contact buttons on the anvil. The chamber 18 had been previously charged with tungsten carbide particles. The particles were obtained from Firth/ Sterling Company of Pittsburgh, Pennsylvania, and are commercially available as Type A. These particles had an irregular surface with an average maximum dimension of about one micron.

Upon metallographic examination, it was found that the carbide particles had penetrated into the gold to a depth of about 100 10- inches and that the concentration of carbide particles was greatest near the surface of the gold.

These reed switch blades were then encapsulated in glass envelopes as shown at 12 and tested. A number of 6 standard acceptance tests were used, and the results uniformly showed the switches made in accordance with this invention to be greatly superior to switches having blades with pure metal contact surfaces.

The superiority of the contact material of the invention is indicated by one test in which the switches were operated at 10 volts and 10 milliamps. On a lot of 32 conventional switches having contact surfaces of electrodeposited gold, 12 samples had failed after 20 million cycles. Of 31 switches made in accordance with the invention, on the other hand, the first failure occurred at million cycles and only two switches had failed at the end of 125 million cycles.

EXAMPLE II Again, the switches made in accordance with this invention performed better than presently used switches with ditfering contact materials. At 10 volts and 10 milliamps, no samples out of 25 failed by 125 million cycles, while of a lot of 32 samples having electrodeposited gold contact surfaces 12 had failed by 20' million cycles.

EXAMPLE III Reed switch blades of the general type shown in FIG. 2 of the drawing were provided with a composite surface in accordance with the invention. The procedure Was the same as that detailed'in Example 1. Samples were tested at 250 volts, 60 milliamps. All of the samples ran for 30 million cycles without a failure.

EXAMPLE IV Electrical relay contact arms of the type shown in FIG. 4 were provided with a composite surface in accordance with the invention. Gold buttons /3" in diameter were machined from solid gold rod with a threaded end. These gold buttons were then sprayed with tungsten carbide by means of the apparatus of FIG. 1. The heated anvil raised the temperature of the contact button to 800 F., the temperature being measured with a thermocouple. Nitrogen gas at a pressure of l-bs./in. was used. The gas was heated by the heating element wrapped around the gas delivery tube (32, FIG. 1). The valve controlling the flow of nitrogen through the tube 22 was opened 4 times to produce 4 spurts of gas directed against the surface of the contact button on the anvil. The chamber 18 had been previously charged with tungsten carbide particles. The particles were obtained from Firth/Sterling Company of Pittsburgh, Pennsylvania, and are commercially available as Type A. These particles had an irregular surface with an average maximum dimension of about one micron. The buttons were then fastened to the contact areas by means of a nut attached to the threaded end of the button.

Two samples were tested on a tester designed to comply with Mil-Spec. No. R6106E (A86). The contacts were operated at 20 volts, 5 amps, with a resistance failure triggering level of 30 milliohms. No failure occurred because of high resistance, and the contacts went an average of 625,000 cycles before welding was noted. Pure gold contacts (presently normally used on this type of relay), exhibited welding at about 100,000 cycles.

EXAMPLE V Three pairs of contact gold buttons in accordance with ASTM test specification B182-49 were provided with implanted tungsten carbide in accordance with the intemperature of the switch. arm to 800 F., the temperature being determined by a thermocouple. Nitrogen gas at a pressure of 50 lbs./in. was used. -In this instance, the gas was not heated.

The valve controlling the flow of nitrogen through the tube 22 vwas opened fourtimes to produce four spurts of gas directed against the surface of the contact button on the anvil. The particles were obtained from Firth/Sterling Company of Pittsburgh, Pa., and are commercially available as Type A. These particles had an irregular surface with an average maximum dimension of about one micron.

Upon metallographic examination, it was found that the carbide particles had penetrated into the gold to a depth of about 100 10- inches and that the concentration of carbide particles was greatest near the surface of the gold.

These', buttons were subjected to testing using an ASTM type arcing contact tester as described in ASTM 3182-49. The three pairs of contact buttons were tested at 110 volts, 10 amps A.C. using a closing force of 100 grams and an opening force of '75 grams: These are the normally used forces for this test. These three pairs of contacts ran an average of 2,225,000 cycles before the first weld. Contact resistance was monitored periodically throughout the test,

and was consistently 0.2 milliohm or below. For comparison pure gold contacts were also run on the same test using the same voltage and amperage, but with a lower closing force because pure gold cold Welds with no current flowing at" the normal closing force. Even then, the pure gold contacts would run only about 50 cycles before the first weld.

Changes in construction will occur to those skilled in the art and various apparently different modifications and embodiments may be made without departing from the scope of the invention. The matter set forth in the foregoing description and accompanying drawings is offered by way of illustrationonlyQ What is claimed is:

1.A method of providing an arc-resistant electrical contact. surface on a current carrying member comprising the steps of:

depositing athin layer of a metal selected from the group consisting of gold, silver, tin, platinum and rhodium on said surface, directing a gaseous stream containing fine arc-resistant particles against said thin layer while said layer is at a temperature in the range of room temperature to 1100 F., said particles having an average maximum dimension in the range of about 20 10- inches to 80x10 inches, the velocity of said stream being suflicient to implant said particles in said layer, and

assembling said current carrying member to an electrical device.

2. A method as set forth in claim 1 wherein said metal is gold and said particles are tungsten carbide particles.

3. A method of providing an electrical contact surface on a current carrying member comprising the steps of:

electrodepositing a layer of a soft conductive metal on the surface of said member until the thickness of the layer is in the range of about 50X 10- inches to 1,000 1O inches,

directing a fluid stream containing fine arc-resistant particles against the surface of said layer while said layer is at a temperature in the range of room temperature to 1100 F., a substantial percentage by weight of the particles in said stream having a maximum dimension in the range of 20 l0- inches to x10- inches and less than the thickness of said 7.

plating, the velocity of said stream and the mass of said particles being suflicient to drive said particles into said layer so that some of said particles are implanted in the surface of said layer, whereby said member is provided with a thin contact surface containing fine dispersed arc-resistant particles, and

assembling said current carrying member to an electrical device.

4. A method of providing an electrical contact surface on a current carrying member comprising the steps of electrodepositing a layer of gold on the surface of said member until the thickness of the gold plating is in the range of about 50x10"- inches to 1,000 X10 inches,

directing a fluid stream containing fine particles of tungsten carbide against the surface of the plated part while said surface is at a temperature in the range of room temperature to 1100 F., the particles in said stream having a maximum dimension in the range of about 20 10- inches to 80 l0- inches and less than the thickness of said plating, the velocity of said stream and the mass of said particles being suflicient to drive said particles into said plating so that some of said particles are embedded in the surface of said plating, whereby said member is provided with a thin contact surface containing fine dispersed tungsten carbide particles, and

assembling said current carrying member to an electrical device.

References Cited UNITED STATES PATENTS 3,211,634 10/1965 Lorenzo 204-16 JOHN H. MACK, Primary Examiner W. I. SOLOMON, Assist-ant Examiner U.'S. Cl. XJR.

29-630 C; 200-166 C; 20435 R, 38 R 

